The invention relates to an energy-absorber for absorbing impact energy.
Within the field of the development of motor vehicles, but also in other fields, passive safety is increasingly becoming an object of central research. In this connection, structures and materials with high energy-absorbing power are of great interest above all. In the automobile industry, for example, use is made of foams consisting of polyurethane (PU) or elastomer-modified polypropylene (EPP). These materials are distinguished by almost ideal behaviour, whereby, after an initially steep increase in force, a horizontal plateau with constant force arises during the absorption of energy which continues to progress temporally. The work that is absorbed in the process is defined as the area under the force-displacement curve; this area should be as large as possible. Various foams that are known from the state of the art approach this ideal quite closely with regard to their energy-absorbing power.
With these energy-absorbers, however, the problem arises that the accelerations which occur are so high that limiting values are exceeded which are not permitted to be exceeded for the impact of a person in the case of road traffic accidents or other accidents. For, in the event of a head impact, an acceleration value of 80 g is only permitted to be exceeded for a time-interval of less than 3 ms.
The plastic deformation of side-member structures may be cited as a further example of energy absorption in the case of a motor vehicle. In this case the metal of the side-member structures is compressed until, under a defined load, the structure collapses, that is to say it buckles, and telescopes inwards. In this case the metal is plastically deformed, resulting in a high absorption of energy.
Although the materials that have been used hitherto for the energy-absorbers result in a high energy-absorbing capacity, the weight of the energy-absorber is not inconsiderable, even when use is made of a light metal such as aluminium. Since weight-reduction plays an important role in the motor industry in particular, the search for extremely lightweight materials with high energy-absorbing power is continuing intensively.
A further reduction in weight is also achieved in the state of the art by the energy-absorbers not taking the form of solid materials but rather by cavity structuresxe2x80x94such as a sandwich structure, for examplexe2x80x94being produced that exhibits a particularly high energy-absorbing power in a preferential direction. For this purpose the cavity structure comprises a plurality of honeycomb chambers that are aligned substantially in the same direction and disposed adjacent to one another. But also in this case it is not possible to remain below a certain weight when use is made of metals or light metals.
The technical problem underlying the invention is therefore to specify an energy-absorber that exhibits both a very low weight and a high energy-absorbing power.
In accordance with the invention it has been recogrised that the moulding is produced from extruded polycarbonate, with the honeycomb chambers extending in the direction of extrusion. In comparison with light metals, polycarbonate is a very lightweight material which is inherently very resilient. In addition, polycarbonate is highly resistant to impact, so that in the event of an abrupt application of force the material does not shatter but deforms elastically and, where appropriate, melts. Consequently the action, according to the invention, as an energy-absorbing material is guaranteed by the polycarbonate as such, with a very low weight of the energy-absorber resulting at the same time.
For production of the moulding, polycarbonate is extruded in such a way that, depending on the extrusion tool that is used, a plurality of honeycomb chambers disposed side by side are formed in the direction of extrusion, each two adjacently disposed honeycomb chambers being separated from one another by a respective common wall. Depending on the size and girth of the moulding, in the course of production several extruded polycarbonate layers with honeycomb chambers are produced which, after extrusion, are connected to one another by material closure. From a block which is consequently formed it is possible for individual sheets to be separatedxe2x80x94with the aid of a hot wire, for examplexe2x80x94which comprise a corresponding plurality of honeycomb chambers, the length of which corresponds to the thickness of the sheet that has been separated from the block.
The honeycomb chambers have a polygonal cross-section, which is preferably either quadrangular or hexagonal. In preferred manner the external dimensions of the honeycomb chambers lie within the range from 1 to 6 mm, it also being possible for these range limits to be transgressed in individual cases. By reason of the small magnitude of the dimensions of the honeycomb chambers the latter may also be designated as capillaries.
Moreover, the honeycomb chambers have a wall thickness within the range from 50 xcexcm to 400 xcexcm. This results in a very low ratio of the wall thickness to the dimension of the honeycomb chamber, which serves as the basis for a further reduction in weight.
The density of the moulding consequently lies, for example, within the range from 30 kg/m3 to 50 kg/m3, which represents a clearly lower value in comparison with energy-absorbers made of light metal.
As has been described above, sheets are cut off from a block which is composed, for example, of several extruded honeycomb-chamber layers, so that the honeycomb chambers pertaining to a sheet have substantially a predetermined length. A moulding of this type may then also be designated as a honeycomb sheet.
Consequently a planar configuration of the moulding of the energy-absorber is also possible.
Besides a flat design of the moulding, the latter may also have a curved shape, in order also to line curved surfaces with the energy-absorber. The moulding is consequently capable of being adapted to a surface, in which connection small radii can also be established, depending on the thickness of the moulding. In every case the honeycomb chambers extend substantially radially in relation to the respective curvature of the surface to be lined. Consequently, impact protection can be guaranteed effectively also in the case of curved inner surfaces.
In another configuration of the moulding at least one of the end faces which include the openings of the honeycomb chambers is provided with a substantially closed layer. Said closed layer is preferably connected to the respective end face by material closure and consequently, besides serving for shaping the moulding in relation to a flat or curved surface, also serves to stabilise the moulding. This layer may also take the form of a sheet or a film and may likewise be produced from polycarbonate or a different synthetic material. It is equally possible for the layer to take the form of a fabric. The layer should preferably be of resilient construction like the moulding, in order not to splinter in the event of an impulse. Furthermore, the layer serves to distribute the application of force onto a relatively large number of honeycomb chambers, since the impact is absorbed not only by the honeycomb chambers that are actually struck without a layer but, by virtue of the covering layer, also by honeycomb chambers disposed in the field surrounding the actual point of impact.
In particularly preferred manner the moulding is arranged on the inner surface of a vehicle, in particular of a motor vehicle, as a result of which the inner surfaces of the vehicle, which as a rule are rigid, are protected. Consequently, in the event of the impact of an occupant the head in particular is reliably protected. By way of inner surfaces in this connection, the pillars that are necessary for the roof construction, the dashboard, and also the inside of the roof enter into consideration.
Furthermore, the moulding previously described may form at least a part of a bumper of a vehicle. By virtue of the extremely lightweight construction, considerable weight-savings in the vehicle can consequently be achieved.
By way of vehicles, both motor vehicles, in particular motor cars, as well as rail vehicles and aircraft enter into consideration. Since a low inflammability of the material is also necessary in addition to the energy-absorbing power, when use is made of polycarbonate the additional advantage arises that polycarbonate is a self-extinguishing material and is accordingly rated in a fire class with low burning capacity. The inflammability of polycarbonate is less than that of other materials employed as energy-absorbers, particularly in the interior of the vehicle, such as polymethyl methacrylate or polystyrene for example.
In another preferred application the moulding may be arranged on the wall of a building. Consequently, in addition to its use in automobile engineering the energy-absorber can also be employed in sports arenas or kindergartens, for example, in order to realise impact protection for people who accidentally strike a wall while engaging in sport. In this connection the use of the energy-absorber for the purpose of damping the floor of a sports venue or a sports hall is also conceivable.
On account of its translucence, polycarbonate is used pre-eminently as the material for structural components that have to exhibit particular optical properties. For instance, polycarbonate is employed for screens and windows that are intended to be translucent. On the other hand, this requirement does not exist in the case of energy-absorbers, so that use can also be made, in advantageous manner, of polycarbonate reject material which, by reason of fabrication defects, is not transparent but is at least partially discoloured and exhibits black or coloured pigments. Consequently, reject material that cannot be used for the production of transparent mouldings can be employed for the production of energy-absorbers.